1. Technical Field
The invention relates generally to spark ignition devices, and, in particular, to spark plugs for internal combustion engines.
2. Background Art
The electrodes of a spark plug are typically made of a material that is resistant to oxidization, heat, and burning. Typical material is a nickel alloy steel and a premium material is platinum. Most spark plugs have two electrodes, a center one 16 and a side one 18, as shown in FIGS. 1 and 2. Between the two electrodes is a physical gap and it is in this gap that a spark is created to ignite the gas mixture in the cylinder of an internal combustion engine and in other burners requiring ignition. The center electrode is connected to the most negative source of the ignition coil while the outer electrode is at ground potential. Thus, relative to one another, the center electrode is a negative electrode and the outer electrode is a positive electrode. The reason for this is that the center electrode is at a higher temperature than the outer electrode. As such, it is a much better emitter of electrons than is the cooler electrode.
Spark plug design is currently a compromise situation. The hotter the center tip, the greater the density of emitted electron and the "hotter" the spark. If it is too hot (e.g., is greater than 1700 Fahrenheit), however, its temperature alone will cause the fuel mixture to ignite before the presence of the spark itself. This is an engine-damaging situation known as preignition or "ping". FIG. 1 illustrates a relatively "cold" plug wherein the electrical insulator 10 is comparatively short thus providing a better thermal cooling path to the outer portions of the spark plug that are in direct contact to the engine head 12. The engine head 12 is, in turn, cooled by water 14 flowing through passages in the head. In the relatively "hot" spark plug shown in FIG. 2, the electrical insulator 10' is comparatively long, thus creating more thermal resistance and allowing the spark electrode tip 16 to become hotter. If the tip 16 is too cool, there will not be a high enough electron density in the spark to properly ignite the fuel/air mixture and the spark plug will eventually foul and become inoperative. The plug tip 16 must be hot enough to preclude fouling, but cool enough to prevent preignition. FIG. 3 illustrates the diametrically opposing constraints. This is further complicated in that temperature changes as a function of engine speed and loading. The present generation spark plugs are optimized for normal highway driving, but are less than optimum for city driving and for high speed driving.
Present generation spark plugs typically have a spark gap of 0.040 inches. The gap is also a compromise. The longer the gap, the higher the probability that the spark will properly ignite the fuel/air mixture and the longer the life of the spark plug as the hot tip will burn away faster when the gap is shorter and the current it emits is higher. The shorter the gap, the higher is the probability of the ignition coil causing a spark to jump between the electrodes and fire the fuel/air mixture, but the shorter gap causes the tip to erode or burn away faster, thus shortening its life. In this wear-out mechanism, the hot tip 16 erodes at a rate about 100 times faster than does the cooler outer electrode 18.
In summary, spark plug design today is a compromise. The hotter the tip, the higher is the probability of a spark jumping the gap between the electrodes 16, 18 under all operating conditions, but the shorter is the operating life of the plug. If it is too hot, however, unwanted ping occurs. If it is too cool, the plug will not fire properly and will soon foul out. The shorter the gap, the higher is the probability that a spark will occur under all operating conditions, but the shorter is the operating life of the plug.
Cesium has long been known to exhibit a Negative Electron Affinity (NEA). This is a situation wherein the energy of the vacuum level is below that of the conduction band electrons on the surface. This enables the material to emit electrons--even when cold. Unfortunately, when exposed to virtually any other element of the periodic table (e.g., oxygen, nitrogen, carbon, hydrogen), the cesium surface is poisoned and it no longer emits electrons.
Recently, aluminum nitride (AlN) and cubic boron nitride (cBN) have been shown to exhibit NEA. Unlike cesium, however, these materials are unusually robust and can be exposed to hydrogen, oxygen, nitrogen, and water and still continue to act as electron emitters. Also, AlN and cBN are very hard materials--much harder than nickel steel alloys or platinum. As such, neither AlN or cBN is easily eroded or burned away as is nickel steel.
Although AlN and cBN are usually insulators, both A1N and cBN can be n-type impurity doped if there is no oxygen present during growth. In contrast to cesium where virtually every element in the periodic table will bind to it with a binding energy greater than does cesium bind to itself (and thus poison its surface), the chemical binding energies in A1N or cBN are very large and virtually nothing will bind to its surface. Only oxygen has been shown to do this and then at an extremely low rate.
Over a long period of time, A1N exposed to atomic oxygen will be converted to sapphire, a crystalline form of aluminum oxide, Al.sub.2 O.sub.3. Fortunately, aluminum oxide is also a NEA material, but it can not be impurity doped and thus has not been used as a cold cathode. Only if the aluminum oxide film is very thin (e.g., less than 20 nanometers) can it be used as a cold cathode electron emitter. The reason for this is that at such thickness, electrons can tunnel through it to the surface where they are emitted into the ambient.
NEA materials will emit electrical current at the same density as does a field emitter, but at an electric field strength of 10,000 times less. Thus much less voltage is required for a given current density when a NEA emitter is used.